change API to use "const char" for strings that are actually constant

[rgerhards]

Unfortunately the API did expose them as char. This is bad from a "clean compile" perspective, but may also convey the idea that these strings are actually modifiable. That is not the case, because it's almost constant string, and so any try to modify them can result in a segfault (depending on platform).

We need to check, though, if fixing this is causing severe trouble. If so, we need to re-evaluate.

[rgerhards]

moving this to 1.2.17 milestone as it requires API user changes, something we do not want to do shortly before rsyslog release; also want to make sure that all existing clients can enjoy the enhancements and fixes in 1.2.16 without change.